Radio communications systems, such as the multi-site trunked land mobile radio system which is described herein as an illustrative example, typically comprise one or more base stations which function as repeaters between mobile radio units. The radio units can communicate in either a direct call mode or a group call mode. In a direct call, one radio communicates directly with another radio through the repeater base station. In a group call, one radio can communicate with a plurality of radios in a designated group. The radio units transmit on one frequency and receive on a different frequency. The base stations act as a repeater between a sending radio and a receiving radio by receiving signals on the sending radio's transmit frequency and re-transmitting the signals on the receiving radio's receive frequency. Such a system is disclosed in U.S. Pat. No. 5,613,201, entitled AUTOMATIC CALL DESTINATION/SYSTEM SELECTION IN A RADIO COMMUNICATION SYSTEM, the disclosure of which is hereby incorporated by reference herein.
The base stations serve a specific geographic area, or cell, and each radio unit is assigned to a specific home cell. The cells may be linked together to cover a large area by linking the base stations so that they can exchange data and transmission signals. This multiple cell arrangement allows two or more radios to communicate even if they are not in communication with the same cell or repeater. The cells monitor assigned radios by tracking which cell is in communication with each radio. Also, each cell maintains a list of all the radios that are communicating with the base station, regardless of whether the radios are assigned to the cell or not.
When a radio unit initiates a call, the calling radio's current base station, or originating cell, performs call set up functions by determining if the called radio is checked into that cell. If the called radio is checked into the cell, then the cell exchanges call set up messages with the called radio. If the called radio is not within the originating cell, then the system determines where the called radio is located and the call information is routed to the called radio's current cell, or the destination cell. The destination cell then exchanges call set up messages with the called radio to complete the call. In the prior art mobile radio communication systems, there is no capability for providing calling radio identification to the called radio. As a result, the called radio users have no way of knowing who initiated the incoming call. The only way to identify the caller in the prior art systems is to actually answer the call. Accordingly, there is a need in the art for a caller identification function which will allow users to determine which radio has initiated a particular incoming call.